Mercury concentration in coal is only at a ppb level and a mercury component exists in a trace amount, but the mercury is emitted to the atmosphere by combustion of the coal and may affect health and the environment, therefore, the mercury should be removed as much as possible from the boiler exhaust gas. FIG. 8A illustrates a flow of exhaust gas treatment of a pulverized coal fired boiler such as a thermal power plant in the related art.
A high-temperature exhaust gas exceeding 1600° C. generated by burning pulverized coal 1 in a boiler furnace 2 reaches 1200 to 1300° C. in the vicinity of an outlet of the boiler furnace 2, and the temperature is decreased to about 400° C. at an outlet 3 of a convection zone of the boiler furnace 2. The exhaust gas discharged from the outlet 3 of the convection zone is subjected to removal of nitrogen oxides (hereinafter occasionally referred to as NOx) by a denitration catalyst filled tank 4 disposed in an exhaust gas flow passage, and is cooled via an air preheater 6 that preheats a combustion air 5 and a heat exchanger (heat recovery unit) 7, and then most fly ash is removed by an electrostatic precipitator (EP) 8 set at about 100 to 200° C. A fly ash 10 discharged from a bottom of the electrostatic precipitator 8 through an ash extraction line 9 is subjected to landfill disposal or sold as a cement raw material, or the like according to properties thereof. The exhaust gas discharged from the electrostatic precipitator 8 is introduced into a wet desulfurization device (desulfurization/absorption tower) 13 by a fan 11 and a desulfurization inlet line 12, and is subjected to removal of sulfur oxides (hereinafter, occasionally referred to as SO2). The desulfurized exhaust gas passes through a mist eliminator 14 to remove scattered mists, and is sent from a desulfurization outlet line 15 to a heat exchanger (reheating unit) 16 to be heated above a dew point, and then is emitted to the atmosphere from a stack 18.
Mercury contained in the coal is vaporized due to a high temperature inside the boiler furnace 2 and becomes a metallic mercury state, but thereafter, is changed to oxidized mercury according to an action of a denitration catalyst and a decrease in the exhaust gas temperature. Apart of oxidized mercury is not only removed from the exhaust gas by the electrostatic precipitator 8 with being caught in fly ash particles, but also dissolved and removed in the desulfurization/absorption liquid in the wet desulfurization device 13, since the oxidized mercury is water-soluble. In recent years, a denitration catalyst having a high mercury oxidation rate has become commercially available, and by applying this catalyst, an amount of mercury dissolved in a wet desulfurization liquid is increased, whereby it is possible to reduce the amount of mercury component emitted from the stack 18 to the atmosphere.
There are various desulfurization methods available in the wet desulfurization device (a desulfurization/absorption tower) 13, however the wet desulfurization method of a limestone-gypsum method illustrated in FIG. 8 is one of the methods with the highest removal efficiency of SO2, and is an excellent technique in which the gypsum of a by-product can be effectively used as a cement material or the like.
A boiler exhaust gas containing SO2 of several hundreds to several thousands ppm introduced into a desulfurization device 13 from the desulfurization inlet line 12 rises in the tower of the desulfurization device 13. Opposite thereto, a slurry of limestone, (main component: calcium carbonate (CaCO3)) contained in the desulfurization/absorption liquid which is collected in a circulation tank 22 at a lower portion of the desulfurization device 13 and is sent via a circulation line 20 by a desulfurization/absorption liquid circulation pump 19, is sprayed from spray nozzles 21 as an absorption liquid to become fine liquid drops. The liquid drop-shaped absorption liquid absorbs SO2 by gas-liquid contact with the exhaust gas to generate calcium sulfite (CaSO3) as shown in the following equation, thereby efficiently removing SO2 from the exhaust gas. At this time, the oxidized mercury in the exhaust gas is also simultaneously dissolved and removed in the absorption liquid.CaCO3+SO2+½H2O→CaSO3.½H2O+CO2 
The absorption liquid dropped in the desulfurization device 13 is collected in the circulation tank 22. The absorption liquid in the circulation tank 22 is stirred by a stirrer 23 at all times, and the calcium sulfite is oxidized by oxygen contained in the air supplied from the air supply line 24 as shown in the following equation to generate crystals of calcium sulfate (CaSO4: gypsum).CaSO3.½H2O+½O2+ 3/2H2O→CaSO4.2H2O
As the concentration of calcium sulfite is decreased from the absorption liquid by oxidation, it is possible to newly absorb SO2, such that the desulfurization efficiency becomes better as the oxidation rate is increased.
It is better for the oxidizing efficiency of sulfurous acid when the pH is low. Meanwhile, if the absorption liquid absorbs SO2, the pH thereof is decreased to reduce SO2 absorbability. However, the absorption efficiency of SO2 may be improved when the pH is high, that is, the concentration of CaCO3 which is alkaline is high. Therefore, in order to satisfy both of the SO2 absorption and the sulfurous acid oxidation, new limestone slurry is supplied from a limestone slurry tank 25 to the circulation tank 22 by a slurry pump 26 and a limestone slurry supply line 27 so that the pH of the absorption liquid is in a range of 5 to 6.
The limestone slurry is prepared by mixing fine powders of limestone and water. The slurry inside of the slurry tank 25 is stirred by a stirrer 28 at all times in order to prevent limestone particles from being settled. The limestone concentration of the limestone slurry in the limestone slurry supply line 27 is usually 20 to 40 percent by weight (wt. %).
A large amount of generated gypsum particles is contained in the absorption liquid in the circulation tank 22. However, by extracting a part of the absorption liquid from an extraction line 30 by an extraction pump 29, the device is usually operated so that the gypsum concentration remains constant within a range of 10 to 30 wt. %. As this will be described below, by dehydrating the extracted absorption liquid, the gypsum is recovered as a by-product that can be effectively used.
If the oxidized mercury dissolved in the liquid is condensed to be a high concentration by circulation of the absorption liquid, the mercury may be re-emitted, and the concentration of mercury in the exhaust gas emitted from the desulfurization device may be increased. Depending on an oxidation state of the absorption liquid, the mercury with a high concentration is often introduced into gypsum crystals. If the concentration of the mercury in the recovered gypsum is increased, there is a problem in terms of effectively using the gypsum. On the other hand, powdered activated carbon is supplied from an activated carbon supply line 31 to the wet desulfurization device 13 to be mixed with the desulfurization/absorption liquid, and the mercury dissolved in the desulfurization/absorption liquid is adsorbed and removed, thereby the mercury concentration in the solution may be suppressed to a low level, as well as re-emission and migration of the mercury to the gypsum may be suppressed.
As a specific method, absorption liquid containing activated carbon after adsorbing mercury and gypsum generated by a desulfurization reaction is extracted from the circulation tank 22 by the extraction pump 29 and the extraction line 30, and is continuously introduced into a foam type flotation device 32. The foam type flotation device 32 generates a large volume of bubbles by supplying air to a gas disperser 33 installed in the vicinity of a bottom thereof by using a pump 34. Since the gypsum is hydrophilic, it is mostly adhered to the bubbles and remains in a dispersed bubble layer 35 under a liquid surface without rising, but since the activated carbon has carbon which is hydrophobic as a main component, it is adhered to the fine bubbles rising from the bottom to further rise, and is collected in the foam bubble layer 36 formed on the liquid surface in a condensed state. As a result, the concentration of the activated carbon in the liquid of the dispersed bubble layer 35 is decreased. Bubbles containing the activated carbon in a high concentration in the foam bubble layer 36 are continuously overflowed from the bubble discharge port 37 provided at an upper portion of the liquid surface, and are recovered by the bubble recovery line 38. The recovered bubbles are damaged by a foam breaker 39 and become a liquid containing activated carbon in a high concentration (referred to as an overflow liquid).
A part of the recovered high concentration activated carbon overflow liquid is returned to the desulfurization device 13 by an activated carbon return line 40 and reused for adsorption of mercury. The remaining part thereof is discharged from the discharge line 41 to outside the system, and is subjected to final disposal such as mercury fixing or mercury recovery. However, since the gypsum content is small, disposal efficiency is good. At the same time, in order to compensate for the amount of activated carbon discharged to outside the system, new activated carbon is supplied to the desulfurization device 13 from the activated carbon supply line 31.
On the other hand, an extraction line 42 is connected to a bottom of the dispersed bubble layer 35 of the flotation device 32, and the absorption liquid having a low activated carbon concentration and high gypsum concentration is extracted by the extraction pump 43. The extracted absorption liquid is dehydrated by a dehydrator (belt filter or the like) 44 to recover the dehydrated gypsum 45. Since the concentration of the activated carbon in the absorption liquid at the bottom of the dispersed bubble layer 35 is low, the obtained dehydrated gypsum has a low mercury content, and becomes a safe material with a high product value without decreased whiteness.
The recovered water after dehydration is stored in a recovery water tank 47 via a line 46. A part of the recovered water is extracted from the recovered water return line 48 by a pump 49, returned to the circulation tank 22 through an on-off valve 67, and reused as makeup water. Depending on the plant, the recovered water may be used for other purposes such as regulated water of limestone slurry. In addition, the remaining part of the recovered water is sent from the drainage line 51 to a drainage treatment equipment 54 through the pump 52 and the on-off valve 53, and is finally treated and then discharged to a river or the sea.
The amount of dehydrated gypsum 45 to be recovered is adjusted so as to be balanced with the amount of gypsum generated by a reaction of SO2 flowing into the desulfurization device 13 and the limestone. In addition, a part of the recovered water is sent to the drainage treatment equipment 54 and discharged to outside the system. Therefore, in order to constantly maintain a liquid level in the circulation tank 22, an amount of the industrial water or the like prepared in a water tank 55 is supplied to the circulation tank 22 by a new replenish water line 56 and a pump 57, such that a concentration degree of the desulfurization/absorption liquid is changed by increasing or decreasing the drainage volume. That is, as illustrated in FIG. 3, the concentration of various ions dissolved in the absorption liquid becomes higher as the drainage volume is lowered. When the concentration of chloride ions in the absorption liquid is too high, the desulfurization performance is decreased, thereby the drainage volume is usually controlled so that the chlorine concentration becomes a predetermined value (for example, 10,000 ppm) or less.
In the above-described prior art illustrated in FIG. 8, there are problems as follows.
Generally, as the bubbles supplied from the bottom are fine, flotation separability of the activated carbon in the flotation device 32 is further improved. Even with the same gas supply amount and the same gas disperser 33, diameters of the generated bubbles vary depending on a composition of a liquid. Therefore, depending on the composition of the absorption liquid, failure occurs in the flotation and separation of the activated carbon, and a recovery rate of the activated carbon due to overflow may be decreased.
The liquid composition mentioned herein is mainly a type and a concentration of the dissolved ions, and the composition of the desulfurization/absorption liquid varies for each plant depending on the type of fuel coal, the type of limestone, properties of water, and operating conditions. Further, even in the same plant, if the properties of the coal, limestone, and water are changed, the composition of the liquid may vary.
If failure occurs in the flotation and separation of the activated carbon in the flotation device 32, the concentration of the activated carbon in the absorption liquid extracted from the bottom is high, but briefly, the activated carbon adsorbing a large amount of mercury is contained in the dehydrated gypsum in a high concentration, thereby causing a problem in effective utilization. In addition, since the activated carbon is black, the whiteness of the recovered gypsum is reduced, and a commercial value thereof may be reduced. On the other hand, if the concentration of the activated carbon in the overflow liquid is reduced, the amount of the activated carbon returned to the desulfurization/absorption tower 13 is decreased, such that the amount of newly supplied activated carbon should be increased, and is therefore uneconomical.
Generally, there may be hydrocycloning as a separation method having a comparatively small influence of the liquid composition on the separability of the solid matter in the absorption liquid. Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2009-61450) discloses a method in which a mercury component in a flue-gas comes into contact with an absorption reagent (a scrubbing solution containing an adsorbent such as activated carbon), thereby the mercury component in the flue-gas is absorbed to the absorption reagent, then gypsum is separated from a desulfurizing liquid by using a hydrocyclone, and the obtained suspension (the adsorption reagent, that is, the activated carbon is present) comes into contact with an oxidation reagent to desorb the activated carbon and the mercury component, thereby removing the mercury compound in the flue-gas. At this time, the activated carbon is separated and removed on an overflow side.
However, as illustrated in FIG. 9, a particle size distribution (solid line and broken line) of powdered activated carbon (with an average particle size of 17 to 19 μm) is finer than that of gypsum in the absorption liquid (dotted line and one-dot chain line), and a particle size range in which the distributions are overlapped is fairly wide. Therefore, the separability of the hydrocyclone is unsatisfactory, and a fairly large amount of the activated carbon is distributed on an underflow side, and mercury is contained in the recovered gypsum. Meanwhile, a part of the activated carbon discharged to the overflow side should be extracted to outside the system and finally disposed of in a form in which the mercury is not eluted. However, since the overflow simultaneously contains a large amount of gypsum, there are problems such as an increase in a final disposal amount.